Coal exists in a wide area of the world. Its minable coal reserve is large, and its price is stable. Thus, the stability of its supply is high, and its price per unit calorific value is low. For this reason, coal-fired thermal power generation plays an important role in ensuring energy and stabilizing energy prices. The coal-fired thermal power generation is also important in its role as an adjuster for the balance between a demand for power by the coal-fired thermal power generation and a demand for power by hydroelectric power generation or nuclear power generation.
Known examples of thermal power generation using coal as a fuel are steam power generation which burns coal in a boiler to obtain steam, thereby driving a steam turbine, and gas turbine power generation which purifies a coal gasification gas, burns the purified gas by a combustor, and expands a combustion gas from the combustor in an expansion turbine (gas turbine) to obtain power by the gas turbine. Combined cycle power generation, for example, is also known in which a steam turbine driven by steam obtained by heat recovery of an exhaust gas from a gas turbine is provided in addition to gas turbine power generation.
As a coal gasification furnace, a fixed bed gasifier, a fluidized bed gasifier, and an entrained bed or flow gasifier are known. As the coal gasification furnace for a power generation plant, an entrained bed type or entrained flow type coal gasification furnace, which melts ash of coal and discharges the molten ash as slag, is mainly developed.
With the entrained flow coal gasification furnace, coal from a burner and an oxidizing agent react within the furnace to reach a high temperature, whereby ash in the coal is melted, and slag-shaped ash is separated. During this process, a gasification reaction proceeds to produce a combustible gasification gas. Part of the molten and slaggy ash may be borne by the gasification gas, and deposited on the inner wall or the like of the furnace. Thus, it has been common practice to supply a quenching gas, which has a temperature lower than the temperature of the gasification gas, to the neighborhood of the outlet of the furnace, thereby lowering the temperature of the gasification gas in order to prevent the deposition of the ash (see, for example, Patent Document 1). As the quenching gas, apart of the resulting gasification gas, for example, is cooled and supplied.
When a part of the gasification gas is used as the quenching gas, it has been necessary to construct a branched path for the gasification gas separated from the slag or the like by a cyclone or the like, and further provide an instrument, such as a heat exchanger, for cooling the gasification gas, or a flow controller for controlling the flow of the gasification gas. Thus, the instruments around the outlet of the gasification furnace have become complicated, the cost of equipment has mounted, and restrictions have been imposed on the design of the shape of the gasification furnace. To keep the efficiency (cold gas efficiency) of the gasification furnace high, it is crucial to maintain the calorific value of the resulting gas at a high level. At the same time, it is also necessary to consider suppressing the deposition of ash.
Thus, it is currently a difficult situation to prevent the deposition of ash, and at the same time, maintain the calorific value of the resulting gas at a high level, while imparting a degree of freedom to the design of the gasification furnace, without changing the design of the gasification furnace.
Patent Document 1: JP-A-9-194854